With Storyprogression off and EA Standard (see game settings) you can only enable witches, vampires, werewolfs and Fairies to be assigned randomly - no mermaids and others.

Airra post_id=92325 time=1671313946 user_id=24734 wrote: mod that immediately assigns 50 sims as supernaturals

I don't think that this is even possible, I guess if the sims are already in town you need to "select" the ones and give occult to them with Mastercontroller only once (other players f.i. may won't like this 50 sims by random for existing sims)
Add Occult and
FILTER Type of Sim (And) -> Sims (the ones without occult) + Residents

for immigration in an ongoing game -> therefor you need to use Storyprogression
When you start a new town f.i. and the world has not many sims

Set the occult town ratio under General - Options:Sims
here ALL types of occult are listed, change the ones you want in your town like to 75 so 75% should be super naturals.
Then of course set all other options under options:lots like I explained above.

If you are done -> set options:lots Rapid immigration --> depending on the size of your families and other settings like single etc.
to 30 (if there are not enough houses, then only the ones possible can immigrate. see also setting Empty lots minimum)

Good luck for the new mod investigation, I am curious, if you will be successfull, then please report

Airra post_id=92325 time=1671313946 user_id=24734 wrote: Do you think it might be easier to make a different mod that immediately assigns 50 sims as supernaturals. That would make it not require an updated story progression. I don't really need random sims to change throughout playthroughs just the beginning. I feel like with my limited knowledge it would just look at an array of sims and select from the array to transform them. Just an idea, but I need to learn how the sims is coded first.

I think you may be underestimating the time it will take to work through the learning curve to accomplish this, and that's assuming you already have C# and .NET coding under your belt. Wouldn't mind being proven wrong on this point, though!

Here is another automated way, more along the lines of what C-Dark was suggesting -- don't play with strictly the EA or other supplied pre-made sims. They may be fun to mess around with the first time, but it's not like they are required to experience the different worlds and the game. Using MC's Total Annihilation, empty the world of all residents except one household first, or maybe remove1/2 or 3/4 of the residents if you want to keep some particular pre-mades around. Then use NRaas SP's Rapid Immigration, with immigration settings that will give you 100%, or maybe 75% if you want some humans, chances of occults in each incoming household. Rapid Immigration will move in up to 5 households every 5-10 sim minutes or something like that until the "order" quantity for RI attempts is satisfied or you run out of suitable empty houses. In a few clicks and really only a few sim minutes you will have a repopulated world with lots of occults to start out with, according to your Town Occult Ratio caps as set here.